Greentext

Regarding the elf slavery, there's a guest post on Rowling's blog (which she must at least have approved even though she didn't write it) that makes the case that Hermione was wrong to attempt to liberate the elves because the elves enjoyed being slaves, and the point of that subplot was to demonstrate that it's bad when people attempt to solve other people's problems without a request for help and bad when people get offended on other people's behalf. A normal person wouldn't let something advocating for such a major misunderstanding of their work on their blog, especially if it was claiming they were pro-slavery.

Most people reading the books would interpret it as implying Hermione was right, but for someone who's seen that blog post on Rowling's site and thinks she read it before it was published and could have vetoed it, it's not a leap to read it again and conclude that it was meant to be pro-slavery and that's why the other main characters, who you're usually supposed to identify with, make fun of Hermione.

On its own, it's simplest to say it just wasn't thought through, but when it's part of a pattern of off-colour opinions, it's harder to give the benefit of the doubt.